I just like all my dice the same or as a set; and I occasionally buy new dice if I like them. I absolutely HATE dice I can't read. Good contrast is a must. I also tend to pick out dice that "fits" my character. Magic user? Depends on the favorite spell, but usually Lightning or Fire. Then I get Crackly Blue (preferably clear/marble type dice) or fiery red with orange speckles. Street Sam? Silverish, Gold, or some metal type dice is a must. if I can't get a metal, I get metallic paints on the pips/numbers. Nature character? I try and find a "natural" set, like from a stone or bone. If not, i get the colors of nature; green generally.
I can really see the mixing of dice with a heavily chaotic character or shapeshifter.